Many shielded electronic equipment enclosures and housings provide internal ventilation, temperature control and ambient pressure equilibration by air circulation through them. Air passage into the enclosures is through electromagnetic radiation shielding devices and air filters located in the enclosure vents.
It is expected of these shielding devices and filters that they prevent, or reduce to acceptable levels, passage of electromagnetic radiation or interference (EMI); and that they also prevent passage of particulate materials, to specified size levels; while minimizing impediments to flow of air through the enclosure. For example, metal honeycomb assemblies are very effective in shielding against EMI, however, they are quite bulky, ranging in thickness from about 1/2" to several inches depending on the electromagnetic radiation frequency range over which they are providing shielding. Furthermore, the honeycomb assemblies are usually arrays of open tubes that provide no barrier properties to passage of dust, dirt, or other particles through them and, consequently, require that separate filtration units be included in the equipment if prevention of passage of particles into the enclosure is important. In general, for installations in which there is great flexibility in space and weight considerations effective EMI-shielding filter assemblies can be constructed from methods and materials well known in the art.
On the other hand, for installations in small, portable electronic devices such as hand-held nuclear radiation detectors, hip-mounted air samplers, and other electronic sensors and instruments or, in mobile electronic systems such as radio/telephone communication systems, missile guidance and control systems, and the like, in which there are severe constraints on the size and weight of components, there are few suitable materials from which compact, lightweight, effective EMI-shielding air filters capable of operating in a broad range of environments can be constructed.